1. Field of the Invention
The present invention relates to devices for contact-free measurement of electrical charge images, and in particular to such an apparatus employing a probe matrix and a means for generating mechanical oscillation of the probe matrix and the charge image relative to each other, the charge thereby electrostatically induced in the individual probes being utilized as an image signal.
2. Description of the Prior Art
Electro-radiographic recording methods wherein a charge image is stored on a high-resistant surface are frequently employed for storing radiation-induced images such as optical images or x-ray images as described, for example, in "Electrophotography," R. M. Schaffert, The Focal Press (1975), pages 191-201. The high-resistant surface may consist, for example, of photoconductive crystals such as a selenium plate. After inital charging, such a plate is modulated during discharge by the incident radiant energy to be imaged. The radiation transmission image is thereby converted into a stationary charge image on the surface of the plate. Another method known as ionography can be used for generating a charge image. Ionization is generated within a volume by the radiant energy to be imaged, and the charged particles are then collected on an electrically insulating surface as a charged image. For generating a visible image, a charged image generated according to the above known methods is subjected to a developing method such as, for example, the application of a coloring pigment powder. Such conventional developing methods, however, have the disadvantage that the charge image on the storage plate is usually lost and further action on the reproduction so as to enhance the reproduction is no longer possible.
Another approach, particularly utilized in medical diagnostics, subjects the electronic charge images on an insulating surface to a high-resolution evaluation before development without the charge distribution, and thus the possibility of producing a direct visible image thereof, being destroyed. It is a problem in the relevant technology to combine electronic image evaluation methods with conventional electro-radiography such as, for example, a combination of computer radiography and electro-radiography as well as pre-development evaluation of optical "xerograohic" methods employing laser printers and the like. If such a combination can successfully be realized an image high in gray scales, for example, could first be electronically produced before the charge image is supplied to a destructive copying process. Computer evaluation of an existing charge image would then also be possible.
A method for non-destructive scanning of charged images by means of an array or matrix comprised of constant potential probes connected to further measuring means which exploits the electrostatic induction caused by the charge image is described in the treatise "Development Of A Large Area Solid-State Image Receptor For X-Ray Images," M. Ein-Gal, Xerox Corporation Technical Annual Report, #NO1-CB-74211-35, October 1978. According to this method, 128 coaxial wires function as contact-free electrostatic induction potential probes. The wires are connected to suitable amplifiers, the output signals of which are subjected to multiplexing and analog-to-digital conversion, and are finally supplied to a computer for processing thereof. Because of the size of the discrete components and the large number of wires, the length and degree of resolution of this known sensor array is inherently limited, thus requiring scanning in an x-y motion pattern. A further disadvantage that this known system exhibits an extreme sensitivity to external influences (microphony sensitivity) because of the interposition of the long wires as well as unstable functioning of the receiver arrangement.
A proposed solution to the problem of non-destructive reading of the electric charge image pattern is described in German Pat. No. 3,121,494 (corresponding to pending U.S. application Ser. No. 377,085 filed on May 10, 1982; L. Risch et al) now U.S. Pat. No. 4,486,778. The apparatus disclosed therein employs a simpler and more precisely functioning measuring means in the form of a matrix of integrated metal-oxide-semiconductor (MOS) components each having a floating gate or a switched floating gate. The elements of the matrix are positioned so as to monitor the local charge condition of the charge image by means of the effect of the electrostatic induction generated by the charge image, which causes the conductivity of the matrix elements to vary accordingly, the measured value being simultaneously amplified.
All of the above measurement systems employ stationary probes wherein a strict dependency of the measured results on the distance from the charged plates exists for a given constant potential measurement for identifying the stationary charge image. This characteristic complicates accurate recognition of nonhomogenious fields and requires high manufacturing tolerances for maintaining a constant spacing of the matrix relative to the surface on which the charge image is generated. Additionally, it is desirable to supress the unavoidable background dc potential by means of superposition of a locally dependent variable potential, however, such is not possible within the framework of the known systems. Lastly, these known systems require high-resistant static electrometer amplifiers which are subject to drift, offset and microphony.